This invention relates to medical ventilators and, more particularly, to an improved system for controlling the positive end expiratory pressure in such ventilators.
In general, medical ventilator systems are used in the administration of anesthesia to a patient undergoing operations and to maintain the patient under anesthesia until the cessation of the operation. Such systems include ventilators to provide a breath to the patient and which typically include a bellows in the system to separate the breathing circuit to which the patient is connected from the drive gas emanating from the ventilator. This is normally done in order to allow the partial reuse of the breathing circuit gases on successive breaths from the patient.
An advantage of such rebreathing in anesthesia systems is that the rebreathing of the gases allows the reuse of the expensive anesthetic agents that are added to such breathing gases. Thus, utilization of the anesthetic agent is reduced and the cost of using such agent is minimized.
There are several types of bellows systems used with medical ventilators including the hanging type of bellows, driven bellows and standing bellows. Of these, the standing bellows is typically driven pneumatically by increasing the pressure within the bellows canister external of the bellows itself during the inhalation cycle by forcing gas from the ventilator into that canister. The bellows is thus forced in a downward direction (gravity added) by the ventilator, thereby expelling the gas from inside the bellows to the patient circuit to breathe the patient. During exhalation, the bellows is allowed to rise back to its original position when the ventilator drive pressure is released and as the patient exhales. Additional fresh gas is admitted to the system to assist in returning the bellows to its full up position.
In order to allow the bellows to rise against the force of gravity, an exhaust valve, commonly known as a pop-off valve, is employed which is biased closed and piloted by the ventilator drive pressure. When that ventilator drive pressure is released, the bellows rises so as to make contact with the top of the bellows canister. At this point, the pressure within the bellows rises rapidly as additional fresh gas is added to the system and reaches the point where the popoff valve is opened against the bias and releases the gases to the atmosphere.
Typically, a relatively small amount of fresh gas flow is continuously added to the breathing circuit so the exhaust of gases through the pop-off valve occurs towards the end of each patient exhalation. The exhaust gas from the pop-off valve is generally scavenged to an appropriate exhaust system in the hospital so that the local area is not contaminated with anesthetic laden gases.
In order to properly ventilate patients exhibiting some degree of respiratory compromise, clinicians frequently use a mode of ventilation called positive end expiration pressure (PEEP). In that mode of ventilation, a positive airway pressure is maintained during the exhalation phase of the patient's breath. Mechanical biased check valves (PEEP Valves) are commonly used within the expiratory side of the patient circuit in order to maintain the PEEP pressure. When the ventilator drive pressure is released, gas exits the pressurized portion of the breathing circuit through the mechanical PEEP valve and the drive pressure thereafter refills the bellows in the normal manner.
For safety and performance reasons, it is desirable to control the PEEP level electronically from the ventilator front panel. One technique for achieving this type of "electronic" PEEP control is to maintain a positive pressure in the ventilator drive circuit during exhalation. One ventilator that produces PEEP in this fashion is shown and described in U.S. Pat. No. 5,315,989 of Tobia, and which is assigned to the present assignee. The disclosure of that patent is incorporated herein by reference. Since the bellows pop-off valve is piloted by the drive pressure, a concomitant rise in the bellows base pressure will also occur. Once the bellows reaches the top of the canister, the pop-off valve bias is overcome and the valve exhausts into a near atmospheric (scavenging) pressure.
One problem with the release of that gas through the pop-off valve under the electronic control condition is that as the gas is released from a relatively high pressure to near atmospheric pressure, too much gas can be released from the pop-off valve causing the pressure within the bellows, breathing circuit and drive circuit to undershoot the desired PEEP pressure level. The cause is due to the pop-off valve relieving across a much higher and variable pressure than occurs without PEEP engaged.
When operating with active PEEP control systems, such as that described in the aforementioned Tobia U.S. Patent, once the PEEP undershoot occurs, the drive pressure is increased in order to reestablish the breathing circuit pressure at the PEEP level. That response serves to shut off the pop-off valve and possibly drive the bellows downwardly slightly. Eventually, the bellows refills with the continuing inflow of fresh gas and once again contacts the top of the bellows canister. As the pop-off valve reopens, the cycle is repeated, creating a sustained "limit cycle" oscillation condition. When operating with a passive PEEP control system, the breathing circuit maintains a constant pressure, but at an indeterminate amount below the PEEP level, also an undesirable outcome.
The aforesaid PEEP control problem can also occur when operating systems are in pressure ventilation mode. Depending on the ventilation parameters set by the clinician and those of the patient, the bellows may refill and engage the top of the bellows canister during the later stages of an inspiratory period. Similar limit cycle oscillations and undershoots to those previously described for PEEP can therefore occur during this portion of the breathing cycle.